I had a little play tonight in a vmware session - liking the omv raid, grow, and online ext4 resizing etc through web interface.
managed to add the xtreemfs repo to apt-get and installed xtreemfs without any major issues, a few hitches with permissions on dirs and where folders are mounted, but looks like it's certainly within the realms of 'do-able'.
For now i'm at least satisfied i 'can' combine OMV & XtreemFS in a single box, even if i need to do a bit of command line work to get it set up in the first instance. Then for drive failures/expanding it can be done online quite easily by any of the other tech staff - even if they don't know linux.